The present invention relates to satellite communications systems, and more particularly to a system using distributed routing and switching systems which minimize the quantity of satellite hardware while achieving the desired objectives of high capacity, highly flexible access, and high reliability for long life.
Still more particularly, the present invention is directed to multiple-pencil-beam, high-capacity satellite systems which provide highly flexible access using a satellite-switched, time-division multiple-access (SS-TDMA) mode of operation with on-board regeneration and processing.
In satellite communications systems, service costs can be reduced by providing distributed access via low-cost earth stations. This is likely to require a proliferation of inexpensive and unattended earth stations that have relatively small antennas and are preferably located at the customer's premises. To compensate for the lower earth station antenna gain, satellite-radiated energy density needs to be increased significantly by concentrating the useful signal energy into a large number of pencil beams having high G/T and high e.i.r.p., rather than spread-out coverage areas such as a hemisphere or the entire globe. The beams may use orthogonal polarization isolation and spatial isolation if necessary. As a result, the overall satellite capacity can be multiplied by reusing the allocated frequency bands at C- and K.sub.u - bands, provided the beam-to-beam co-channel interference can be minimized. This can be achieved by on-board regeneration of all the digital signals, since demodulation and remodulation effectively separate performance degradations of the up- and down-links, and only result in the algebraic additions of bit-error rates.
These improvements, however, require more sophisticated SS-TDMA earth station equipment and a highly flexible means on board the satellite for interconnecting all the earth stations located in different coverage areas or beams. See, for example, F. T. Assal, R. Gupta, J. Apple, and A. Lopatin, "A Satellite Switching Center for SS-TDMA Communications," COMSAT Technical Review, Vol, 12, No. 1, Spring 1982, pp. 29-68; S. J. Campanella and R. Colby, "Network Control for TDMA and SS-TDMA in Multiple-Beam Satellite Systems," Fifth International Conference on Digital Satellite Communications, Genoa, Italy, March 1981; and T. Inukai, "An Efficient SS-TDMA Time Slot Assignment Algorithm," IEEE Transactions on Communications, Vol. COM-27, No. 10, October 1979, pp. 1449-1455. Compared to global coverage satellites, multiple-narrow-beam satellites complicate the overall system architecture by requiring flexible interconnections, possibly among all the participating earth stations. Microwave switch matrices (MSMs) have been proposed for satellite-switched, time-division multiple-access (SS-TDMA) in INTELSAT VI, as described in S. B. Bennett and D. J. Braverman, "INTELSAT VI--A Continuing Evolution," Proc. IEEE, Vol. 72, November 1984, pp. 1457-1468, and it is anticipated that regeneration, baseband switching, and/or processing will be introduced on-board NASA's Advanced Communications Technology Satellite (ACTS), as described by W. Holmes and G. Beck, "The ACTS Flight Segment: Cost Effective Advanced Communications Technology," AIAA 10Th Communications Satellite Systems Conference, Orlando, Flor., March 1984, and in Italy's ITALSAT, as described by S. Tirro, "The ITALSAT Pre-Operational Program," Sixth International Conference on Digital Satellite Communications, Phoenix, Ar., September 1983. Thus, the simultaneous operation of a large number of pencil beams becomes a feasible and attractive means of reducing communications service cost. With regeneration, the number of simultaneously operating beams can be increased significantly, since beam-to-beam co-channel interference is limited to up- and down-links, separately.
Another important aspect of communications system design is hardware redundancy. To ensure communications hardware survivability in space, redundancy (2-for-1, or 3-for-2) is usually provided for all active subsystems, that is, for the low-noise receivers (LNRs) and the high-power amplifying transmitters. Depending on the requirements, enhanced satellite reliability is achieved by pooling failure-prone devices such as the frequency-channelized power transmitters, to produce a "double-ring" redundancy network, as shown in FIGS. 1 and 2. See, for example, F. T. Assal, C. Mahle and A. Berman, "Network Topologies to Enhance the Reliability of Communications Satellites," COMSAT Technical Review, Vol. 6, No. 2, Fall 1976, pp. 309-322. In this hypothetical example, a 12-for-8 double-ring redundancy is provided for enhanced reliability. Any 4 of the 12 transmitters could fail without affecting the operation of eight channels, because the RF signals would be routed to the surviving amplifiers.
The present invention is directed to improvements in multibeam systems. However, to place the present invention in proper perspective, a brief review of the advantages of global beam satellites will first be provided, followed by a discussion of multiple-beam configurations that use fixed, scanning, and hopping beam techniques which allocate critical satellite resources by inner switching, outer routing, or outer switching, respectively.
Global beam coverage satellite with no routing/switching, receiving communications signals at 6 GHz and retransmitting a 4 GHz, provide for the simplest and most flexible operational systems. Such systems were used in early INTELSAT satellites and are currently provided as complementary packages in multiple-beam systems such as INTELSAT V and VI. A single beam covering the 18.degree. field of view of the earth is generated using a single horn with an edge-of-coverage gain of about 17 dBi. As a result, low G/T and low e.i.r.p. require large earth station antennas. Also, the allocated frequency band (500 MHz in the up-link and 500 MHz in the down-link) may be reused only once, thereby limiting the overall available capacity. Since the earth stations could receive all the satellite transmitted signals, this single-beam coverage provides full connectivity in either frequency-division multiple-access (FDMA) or TDMA.
Multiple spot beams can be implemented as fixed, hopping, and/or scanning beams. Fixed SS-TDMA beams are configured (as is being done for a relatively small number of beams in INTELSAT VI and ITALSAT) once it is assumed that the traffic capacity projections are fairly accurate and that each beam contains sufficient capacity to warrant the dedication of scarce power resources to the illuminated areas.
In a fixed-multibeam system with inner routing/switching, all of the beams are basically frozen, critical satellite resources are permanently assigned to each beam, and signal inner-routings or switching cannot reallocate these resources.
Conceptually, flexibility of operation in an on-board processing SS-TDMA satellite system can be provided by using multiple scanning beams. In the case of scanning systems, the narrow beams formed by using an active phase-array antenna are steered on demand to focus the radiated power on specific areas. FIG. 4 depicts a large number of spot beams (e.g., about 100 for 1.0.degree. beams or about 400 for 0.5.degree. beams) covering land masses within the AOR global coverage area visible from geostationary orbit. To significantly increase the communications capacity and flexibility of access within the SS-TDMA frame, adjacent beams may be scheduled to have different frequencies of operation, and multiple frequency reuse can be implemented for spatially isolated beams. As the beams are narrowed, the resulting increase in satellite antenna gain, with a concomitant increase in G/T and e.i.r.p., makes possible distributed access via low-cost customer-premises earth stations.
An alternative technique of obtaining flexibility is a multiple hopping beam system. The hopping-beam configuration contains a full complement of antenna elements, low-noise amplifiers (LNAs), and power transmitters, which are energized on demand only when required for transmissions. Single feed elements, in conjunction with a focal-region-fed optical system, are used to generate a number of pencil beams for the multiple-hopping concept. A spherical wave radiating from the single feed element (or from a cluster of very few feeds) is transferred through a single- or dual-reflector system to produce a plane wave in the desired direction of the beam. The single offset reflector configuration is the simplest and currently the most widely used for communications satellites, while a dual-reflector system produces less scan loss and better co- and cross-polarization isolation. Recently, it has been shown that a side-fed offset Cassegrain system offers the best overall performance among the dual-reflector configurations. See, for example, R. Jorgensen, P. Balling, and W. J. English, "Dual Offset Reflector Multibeam Antenna for International Communications Satellite Applications," IEEE Transactions on Antennas and Propagation, Vol. AP-33, December 1985, pp. 1304-1312.
Turning attention specifically to the Intelsat system, the primary motivation in designing the INTELSAT I through VI series of satellites was to satisfy an increasing demand for communications capacity to and from large gateway or national earth stations (i.e., a capacity increase of from 240 to 33,000 telephone channels per satellite). To meet this capacity demand, INTELSAT VI was designed to reuse the allocated frequency bands by focusing its radiated energy into two and six isolated beams at K.sub.u - and C-bands, respectively. At K.sub.u -band, two spatially isolated spot beams are provided having approximately 40 dBi of gain, while at C-band there are four spatially isolated zones in one polarization sense with approximately 30 dBi of gain and two spatially isolated hemispherical beams with the opposite sense of polarization with approximately 23 dBi of gain.
To include areas not covered by any of these beams, two orthogonal earth coverage (global) beams having approximately 17 dBi or gain utilize about 120 MHz of allocation bandwidth at C-band. FIG. 3 illustrates the coverage areas of these beams for the Atlantic Ocean Region (AOR). At C-band, the zone and hemi beams have opposite circular polarization, while at K.sub.u -band the spot beams have orthogonal linear polarization. To accommodate the different locations of population centers in the Indian and Pacific Ocean Regions, two complex sets of mechanical switches are provided in the satellite antenna subsystems at C-band. Since the primary mode of transmission is FDMA, path-to-path interconnections are easily achieved with a set of semi-static mechanical switches for frequency multiplexed channels having bandwidths of 36 to 72 MHz. To increase the flexibility of operation and access, two (6.times.6) MSMs are introduced for SS-TDMA in two 72-MHz channels. In either case inner-switching, and hence a fixed-beam satellite concept, is employed.
Although the beams are designed to be isolated, sidelobes and imperfect polarization result in a system that is limited by co-channel interference. As communications capacity requirements continue to increase beyond INTELSAT VI for the primary satellite (i.e., the satellite which will permit all earth stations located within the global coverage area, as seen from geosynchronous orbit, to communicate with each other), it may be necessary to demodulate and remodulate digital signals, thereby increasing the number of frequency reuses by providing a larger number of narrow, pencil-like beams. The use of on-board regeneration with hard-decision isolates up- and down-link impairments and allows each transmission path to be optimized separately. In the up-link, these impairments include the temperature of the earth, thermal noise (primarily generated in the receiver), and adjacent beam-to-beam co-channel interference. Similar effects occur in the down-link.
On-board data regeneration is described in S. J. Campanella, F. T. Assal and A. Berman, "On-Board Communication Processing Technology," Symposium on Transportation and Communications, Genoa, Italy, October 1980; S. J. Campanella, F. T. Assal and A. Berman, "On-Board Regenerative Repeater," IEEE International Conference on Communications, Chicago, Ill., June 1977, Conference Record, pp. 6.2-121 to 6.2-125; and Y. S. Lee, "Simulation Analysis For Differentially Coherent Quaternary PSK Regenerative Repeater," COMSAT Technical Review, Vol. 7, No. 2, Fall 1977, pp. 447-474. On-board data regeneration results in the addition of up- and down-link bit error rates (BERs), whereas a nonregenerative satellite causes up-link impairments to be passed to the down-link. Therefore, on-board regeneration allows for a significant reduction in the up- and down-link energy-per-bit to noise-power-density ratios (E.sub.b /N.sub.o) in order to achieve the same BERs. Alternatively, the desired performance can be achieved, even with several co-channel interferers, by using several "isolated" beams operating simultaneously in the same frequency bands.
An on-board processing multi-beam satellite configuration will be described with reference to FIG. 4, which shows a simplified block diagram of a fixed-multibeam, on-board processing satellite for SS-TDMA operation. This representative example comprises N up- and down-link beams. Using conventional techniques, the allocated frequency band for each of the N fixed beams is subdivided or demultiplexed into M channels. Up to (M.times.N) up-link TDMA signals are received, low-noise amplified, demultiplexed into separate (1-to-M) channels, and then demodulated. Assuming quaternary phase-shift keying (QPSK), the output of each demodulator includes the recovered clock component and the regenerated I and Q bit streams.
Inner-routing/switching and on-board processing precede remodulation, power amplification, multiplexing, and retransmission of the regenerated carriers through a set of N fixed beams. Assuming that some beams are only lightly loaded, it is wasteful of satellite resources to provide these beams with M channels. Hence, once a channel is assigned, bit rate selection for transmission to and form these low-capacity beams severely restricts the participating earth stations. In addition, flexibility in selecting carrier sizes, as provided in nonregenerative FDMA systems, is no longer possible. Therefore, the limitations of inner-routing or switching are inconsistent with the marketplace, which is driving satellite system architectures toward greater efficiency and cost consciousness while demanding highly flexible access to a growing number of services and customers.
As the number of beams is increased in order to increase G/T, e.i.r.p., and the number of frequency reuses, most of the areas covered by the narrow beams do not need to be illuminated all the time. Therefore, hopping and scanning beams provide additional flexibility by assigning the available RF power in short, cyclical intervals on demand.